While phase locked loop design in accordance with the present invention is applicable to any device utilizing a phase locked loop, it is particularly adapted for radio frequency communication devices in which frequency conversion of a base band signal to the RF (radio frequency) transmission frequency or vice versa is performed in two frequency conversion steps. Using the transmission path of a cellular telephone as an example, a cellular telephone will frequency up-convert a base band signal to the RF transmission frequency, in two frequency mixing (heterodyning) steps. Depending on the particular country and the particular band, the RF transmission frequency for cellular communications may be in the range of about 800 MHz-900 MHz or about 1800 MHz-1900 MHz.
Commonly, a base band signal will first be frequency up-converted to an intermediate frequency (IF) of, for example, about 270 MHz by mixing the baseband signal with an IF reference signal generated at 270 MHz. The IF information signal generated thusly is then frequency up-converted to the RF transmission frequency by mixing it with an RF reference signal generated by a second PLL.
FIG. 1 is a block diagram of an exemplary cellular telephone frequency conversion circuit 10 of the prior art. Both the transmit 11 and receive 13 paths are shown. This circuit is found, for instance, in the W3020 and W3000 chips manufactured by Lucent Technologies, Inc. of Murray Hill, N.J., U.S.A. An incoming differential base band signal, represented in the figure as differential signals TX.sub.I, and TX.sub.Q, are fed to mixers 12 and 14, respectively. TX.sub.Q is the real part of the signal and TX.sub.I is the imaginary part (phase shifted 90.degree. from the real part) of the signal. Mixers 12 and 14 mix the incoming signals TX.sub.I and TX.sub.Q, respectively, with a signal generated by a first local oscillator 16 to perform the first frequency up-conversion on the data signal from the baseband to the intermediate frequency. Local oscillator 16 is, for example, a PLL generating a signal at 540 MHz. That signal is forwarded to a divider circuit 18 which frequency divides the oscillator signal by two or by three depending on the frequency band of operation. Particularly, this circuit is adapted for a dual-band cellular telephone and therefore selectively provides two separate intermediate frequencies, namely, 180 MHz and 270 MHz, depending on the final RF frequency to be achieved. The IF reference signal from the divider 18 is forwarded to phase quadrature circuitry 20. The phase quadrature circuitry creates two signals at the IF reference frequency 90.degree. out of phase with each other and forwards them to the mixers 12 and 14, respectively. The mixers 12 and 14 mix the real and imaginary parts of the base band signal with the real and imaginary portions of the IF reference frequency, respectively, to create real and imaginary signals 22 and 24 at the intermediate frequency. An adder 26 sums the signals to create a complete signal on line 28 at the intermediate frequency containing the information in the original base band signal. That signal is filtered by a band pass filter 30 to eliminate noise and harmonics created by the mixers 12 and 14 and is forwarded to a second mixer 32. Mixer 32 is the RF frequency mixer which will frequency up-convert the IF frequency information signal to the RF transmission frequency. RF mixer 32 mixes the intermediate frequency signal on line 33 with a signal generated by a second local oscillator 34 at an RF frequency displaced 270 MHZ from the desired transmission frequency to generate a side band signal at the desired RF transmission frequency. This local oscillator circuit 34 comprises two alternately selectable PLLs 34a and 34b since many countries, including the U.S. and European countries, have two broad frequency bands within which cellular communications are permitted. Accordingly, a local oscillator is provided for each broad frequency band. Within each of the broad bands is a series of narrower frequency channels from which each cellular telephone will use one channel for a given call. Switch 36 will select the signal from one of the local oscillators depending upon the selected cellular broad band and forward it to amplifier 38. The output of amplifier 38 is fed to the second input of the RF mixer 32 to mix the intermediate signal on line 30 with the RF local oscillator reference signal to create an RF information signal on line 33. That signal is filtered further by band pass filter 43 to eliminate harmonics and background noise, amplified by amplifiers 48 and 50 and forwarded to the antenna 52.
The receive path circuitry 13 is basically the same circuitry in reverse. It comprises a pair of filters 60 and 62, respectively, tuned to the two broad bands permitted for cellular telephone communications by the particular country. Those signals are amplified by low noise amplifiers 64 and 66, respectively, and forwarded to additional filters 68 and 70, respectively. Those signals are mixed by mixers 72 and 74, respectively, with the signals generated by the RF frequency PLL 34a or 34b at a frequency that is displaced 270 MHz from the received RF signal, respectively, to create a side band signal at an intermediate frequency of 270 MHz. Up to this point, both paths (i.e., through elements 60, 64, 68 and 72 and through elements 62, 66, 70 and 74) process the received signal regardless of which broad band it is within even though the output of only one of the paths (the one that is tuned to the frequency of the particular incoming signal) will be used. The outputs of both mixers 72 and 74 are filtered by filter 76 and forwarded to an amplifier 78. Filter 76 not only cleans up the signal, but also acts as a selector of the signal from the appropriate path. Specifically, it will pass only the signal in the path that was tuned for the RF frequency of the particular received signal. The signal sent to the filter that was generated in the other path will not be at 270 MHz because it was mixed with an RF reference frequency that was not offset therefrom by 270 MHz.
The intermediate frequency signal on line 79 is mixed by mixers 80 and 82 with the 270 MHz reference signal generated by the IF PLL 16 as divided by divide by two divider 84 and broken into two components phase shifted 90.degree. from each other by phase differentiator 86. This separates the signal into its real and imaginary parts. The outputs are filtered by filters 88 and 90, respectively, amplified by amplifiers 92 and 94, respectively, filtered further by filters 96 and 98, respectively, and amplified again by amplifiers 100 and 102, respectively. The signals are then passed on to base band processing circuitry.
Each local oscillator 16, 34a, and 34b is comprised of a phase locked loop (PLL) for generating a well regulated frequency signal. A typical PLL includes at least a phase comparator, a voltage controlled oscillator, a charge pump, a feedback loop, an oscillator, filters, amplifiers, and multiple frequency dividers.
The circuit of FIG. 1 requires an IF PLL 16 and a separate RF PLL (in this case a pair of RF PLLs 34a and 34b).
FIG. 2 is a block diagram of a typical phase locked loop circuit that can be used for any of the local oscillator circuits 16, 34a or 34b in FIG. 1. For exemplary purposes, we will consider it to be used for the local oscillator 16 for frequency up-converting the base band signal to an IF signal centered at 270 MHz. The circuit comprises a crystal oscillator 202 generating an oscillating signal at a frequency of 13 MHz. The output of the crystal oscillator is provided to a divide by 13 circuit 204 to generate a 1 MHz signal on line 206. That signal is provided to the first input of a phase comparator 208. The second input of the phase comparator is coupled to the feedback signal 209. The feedback signal is essentially a conditioned version of the frequency controlled output signal of the PLL.
The phase comparator functions in the feedback loop to make the phase (and thus the frequency) of the two signals 206 and 209 presented at its inputs equal by regulating its output accordingly. Specifically, when the phase of the feedback signal 209 leads the phase of the reference signal 206, phase comparator 208 outputs a positive voltage pulse. When the phase of the feedback signal 209 lags the phase of the reference signal 206, phase comparator 208 outputs a negative voltage pulse. If the phases are equal, it generates no signal.
The output of the phase comparator 208 is supplied to a low pass filter 210 which averages the phase difference signal to create a DC average voltage output on line 212. That output is provided to a voltage controlled oscillator (VCO) 214 which increases the frequency of its output in response to a positive voltage and decreases the frequency of its output in response to a negative voltage to generate a signal having a frequency that is a function of the input voltage on line 212. The feedback loop is tuned (by use of dividers) such that the output of the VCO will be a signal having a frequency of 270 MHz when the phases of the reference signal 206 and feedback signal 209 are equal at 1 MHz. Accordingly, the VCO output signal on line 215 is the PLL output signal at the desired frequency of 270 MHz. In order to establish 270 MHz as the PLL's output frequency, this signal 215 is fed back through a divide by 270 circuit 218 to the second input of phase comparator 208. Accordingly, the operation of the feedback loop maintains the output of the VCO at 270 MHz.
It is desirable to reduce the complexity of the overall communications circuit and particularly to reduce the number of local oscillator circuits required to provide the necessary functionality.